This invention relates to a laser thermal (heat) transfer recording method whereby a full-color image with a high resolution is formed using laser beams. More specifically, it relates to a laser thermal transfer recording method which is useful in forming a color proof (DDCP: direct digital color proof) or a mask image by laser recording derived from digital image signals in the filed of printing.
In the field of graphic art, a printing plate is baked with the use of a set of color separation films constructed from an original color copy using lith films. Prior to the main printing (i.e., the practical printing procedure), it has been a practice to form a color proof from the color separation films in order to check errors in the color separation step and examine the necessity for color correction, etc. It is desired that such a color proof has a high resolution power so as to allow high reproducibility of medium contrast images, a high process stability and the like. To obtain a color proof closely similar to the actual printed matter, it is preferable to employ the same materials as used in the actual printed matter (for example, printing paper as a base material and pigments as colorants) in the color proof. To form a color proof, it is also highly desirable to use a dry method without resort to any developers.
As a dry method for forming a color proof, there has been developed a recording system wherein a color proof is directly formed from digital signals with the recent popularization of the electronic systems in the pre-printing step (in the pre-press industry). These electronic systems aim at, in particular, forming color proofs with high image qualities. In general, dot images of 150 lines per inch or above can be reproduced thereby. To record a proof with high image qualities from digital signals, use is made of, as a recording head, laser beams which can be appropriately modulated depending on digital signals and by which recording beams can be finely stopped down. Accordingly, it has been required to develop a recording material having a high sensitivity to laser beams and showing a high resolution power enabling the reproduction of highly fine dots.
As recording materials usable in the transfer image-formation method with the use of laser beams, there are known hot-melt transfer sheets having a photothermal conversion layer, which absorbs laser beams and generate heat, and an image formation layer, in which a pigment is dispersed in other components such as a hot-melt wax and a binder, on a substrate in this order (JPA 5-58045). In the image formation method using these recording materials, the image forming layer is molten in the parts corresponding to the heat generated from laser-irradiated region of the photothermal conversion layer and thus transferred onto the image receptor sheet provided on the transfer sheet. Thus the transferred image is formed on the image receptor sheet.
JPA 6-219052 discloses a thermal transfer sheet having a photothermal conversion layer containing a photothermal conversion substance, an extremely thin heat removable layer (0.03 to 0.3 xcexcm) and an image forming layer containing a colorant on a substrate in this order. In this thermal transfer sheet, the binding force between the image formation layer and the photothermal conversion layer mediated by the above-described heat removable layer is weakened by irradiation with laser beams and thus a very fine image is formed on the image receptor sheet provided on the thermal transfer sheet. The phenomenon so-called xe2x80x9cabbrationxe2x80x9d is utilized in the image formation method with the use of the above-described thermal transfer method. More particularly speaking, the heat removable layer is partly decomposed and vaporized in the laser-irradiated regions. As a result, the adhesiveness between the image receptor layer and the photothermal conversion layer is weakened in these regions and thus the image receptor layer in these regions is transferred onto the image receptor sheet laminated thereon.
These image forming methods have advantages such that a printing paper having an image receptor layer (an adhesive layer) can be used as an image receptor sheet material, a multicolor image can be easily obtained by successively transferring images with different colors on the image receptor sheet, and a highly fine image can be easily obtained. Therefore, these methods are useful in forming color proofs (DDCP: direct digital color proofs) or highly fine mask images.
To shorten the recording time in recording an image using laser beams, laser beams consisting of multibeam with the use of a plural number of laser beams are employed in recent years. In case of recording an image with the use of an existing thermal transfer sheet with multibeam laser beams, there sometimes arises a problem that the transferred image has only an insufficient density. A particularly serious decrease in image density is observed in high-energy laser recording. As the results of examinations by the present inventor, it has been clarified that such a decrease in image density is caused by uneven transfer occurring in case of high energy laser irradiation.
In the above-described recording methods, use is made of one image receptor sheet R and a plural number of thermal transfer sheets such as K (black), C (cyan), M (magenta) and Y (yellow). In recording media, it has been a practice to laminate 20 to 100 sheets of the same type and package. In case of packaging about 25 sheets as shown in FIG. 10, for example, recording media 1 of the same type are vacuum-packaged in a packaging material 3 such as a synthetic resin bag made of, for example, polyethylene and further packed in a decorative box 5 made of corrugated fiberboard or the like to give a package 7.
Prior to setting in a recorder, five types of such packages 7, i.e., an image receptor sheet R and thermal transfer sheets K, C, M and Y are opened. The recording media thus opened are manually set into a recording medium cassette of the recorder in the reverse order to the recording order. That is to say, the thermal transfer sheet Y is first taken out from the packages 7 having been opened and set into the cassette. Subsequently, the thermal transfer sheets, M, C and K and the image receptor sheet are similarly set into the cassette. Thus, a plural number of recording media consisting of the image receptor sheet and the thermal transfer sheets K, C, M and Y (from top to bottom) are laminated and set in the cassette. In case of setting a plural number of recording medium sets, the above-described procedure is to be repeated.
Since recording media of respective types are separately packaged in existing packages, one recording medium should be taken out from each of the packages of the image receptor sheet R and thermal transfer sheets K, C, M and Y having been opened and set into a cassette. Therefore, individual recording media are exposed to the outer surroundings and thus the possibility of the adhesion of foreign materials is elevated. The adhesion of foreign materials brings about a problem that printing cannot be normally carried out and there arise defects such as white spots and uneven ring pattern.
Moreover, the individual recording media should be manually set into the cassette in the reverse order to the printing order. Thus, there frequently arises a problem that the order of color recording is mistaken due to an error in setting.
Furthermore, the image receptor sheet or the thermal transfer sheets should be picked up from the recording medium cassette and transferred into the recorder using a picking up system such as a rubber roller or a sucking/adsorption system. During this operation, there arises another problem of positioning error or jamming.
Under these circumstances, the present invention aims at providing a laser thermal transfer recording method whereby an image receptor sheet or thermal transfer sheets can be transported and fed in a stable state without causing jamming or positioning error to thereby give an image free from any defects in the image caused by the adhesion of foreign materials or mistaken color recording order due to an error in manual operation.
The above problem can be solved by the following means.
1. A laser thermal transfer recording method which comprises the steps of feeding an image receptor sheet having an image receptor layer and a plural number of thermal transfer sheets having at least a photothermal conversion layer and an image formation layer on a substrate from a recording medium cassette, superposing the image receptor layer of the above-described image receptor sheet upon the image formation layer of the above-described thermal transfer sheets and holding them on a recording medium support member, and then irradiating the above-described thermal transfer sheets with laser beams appropriate for image data to transfer the laser-irradiated regions on the image formation layer onto the image receptor layer of the above-described image receptor sheet thereby recording an image, characterized in that the above-described image receptor sheet and the above-described thermal transfer sheets are laminated in the order of feeding into the recording medium support member and contained in the above-described recording medium cassette and the coefficient(s) of static friction of the back layer surface of the above-described image receptor sheet and/or the above-described thermal transfer sheets are 0.7 or below.
2. The laser thermal transfer recording method according to the above-described 1 characterized in that a package, which has the above-described image receptor sheet and the above-described thermal transfer sheets laminated in the order of feeding into the recording medium support member and packed therein, is opened and then the thus laminated image receptor sheet and thermal transfer sheets are set in the above-described recording medium cassette at once.
3. A laser thermal transfer recording method according to the above-described 1 or 2 characterized in that the coefficient of static friction of the image receptor layer surface of the above-described image receptor sheet is 0.5 or below.
4. A laser thermal transfer recording method according to any of the above-described 1 to 3 characterized in that the surface roughness Rz of the image receptor layer surface of the above-described image receptor sheet is from 1 to 5 xcexcm.
5. A laser thermal transfer recording method according to any of the above-described 1 to 4 characterized in that the surface roughness Rz of the back layer surface of the above-described image receptor sheet is 3 xcexcm or below.
6. A laser thermal transfer recording method according to any of the above-described 1 to 5 characterized in that the surface electrical resistance SR of the image receptor layer surface of the above-described image receptor sheet is 1014xcexa9 or below when measured at 23xc2x0 C. under 55% RH.
7. A laser thermal transfer recording method according to any of the above-described 1 to 6 characterized in that the surface electrical resistance SR of the back layer surface of the above-described image receptor sheet is 1012xcexa9 or below when measured at 23xc2x0 C. under 55% RH.
8. A laser thermal transfer recording method according to any of the above-described 1 to 7 characterized in that the coefficient of static friction of the image formation layer surface of the above-described thermal transfer sheets is 0.5 or below.
9. A laser thermal transfer recording method according to any of the above-described 1 to 8 characterized in that the surface roughness Rz of the image formation layer surface of the above-described thermal transfer sheets is 3 xcexcm or below.
10. A laser thermal transfer recording method according to any of the above-described 1 to 9 characterized in that the surface roughness Rz of the back layer surface of the above-described thermal transfer sheets is 7 xcexcm or below.
11. A laser thermal transfer recording method according to any of the above-described 1 to 10 characterized in that the surface electrical resistance SR of the image formation layer surface of the above-described thermal transfer sheets is 1011xcexa9 or below when measured at 23xc2x0 C. under 55% RH.